1. Field of the Invention
The present invention relates in general to a method for recording information on optically sensitive media, and more particularly to a method suitable for recording information on proofing and lithographic printing plates.
2. Description of Related Art
Traditional techniques of introducing an image onto recording media include photoresist, color proofing, letterpress printing, flexographic printing, gravure printing and offset printing. These traditional techniques use various light sources to expose photosensitive materials with analogue lithographic masking film technology. The quality of exposure contributes to the resolution, durability and overall condition of the image in the final products.
Digitally modulated lasers are typically used to expose recording media. Lasers have been the preferred light source for digital imaging devices because they provide collimated, monochromatic emission. Collimated single wavelength light beams are easy to modulate and do not suffer from chromatic aberrations, thus eliminating the need for sophisticated achromatic optics. Collimated single wavelength light, however, has some drawbacks in the application of recording information on optically sensitive media.
Most photosensitive recording materials form three-dimensional layers with a discrete thickness. For example, in offset printing plates the photosensitive materials are typically coated as layers 0.5-10 micron thick on a substrate, and flexographic printing plates are coated or extruded as a 1-7 mm thick layer on a substrate. Although the recording media is a three-dimensional layer, the monochromatic light sources typically used to photo-expose the recording media generate a substantially two-dimensional focal point, as schematically shown in FIG. 1. A laser light source 14 is focused through a chromatic lens 16 onto a photosensitive layer 12. The monochromatic light is focused to a two-dimensional focal point 18 at the surface of the photosensitive layer 12. A two-dimensional single focal point optimally exposes only one level in the three-dimensional photosensitive layer. In practice, the two-dimensional focal point is typically projected at the surface of the layer of photosensitive material, and the photosensitive material below the surface is not sufficiently photo-exposed.
There are many problems associated with insufficient photo-exposure below the surface of the photosensitive layer.
Insufficient photo-exposure results in inadequate initiation of the photochemical reaction in the recording medium. The photochemical reaction cures the recording medium and the reaction is the foundation for all the subsequent processes in printing applications. The practical implications of insufficient photochemical reaction in the recording material include poor printing run length, poor ink receptivity, side etching and poor bonding to the substrate.
Since photo-exposure of recording media is so critical in printing applications, many adaptations have been made to optimize the photo-exposure process. One adaptation is the use of short wavelength light sources. Short wavelength radiation has higher energy to initiate the photochemical reaction in recording materials. Shorter wavelengths of light also provide higher resolution capability than longer wavelengths. However, short wavelength radiation does not penetrate well below the surface of the photosensitive material. Shorter wavelengths of light reflect off the surface of recording media more than light of longer wavelength. Higher reflectance off the surface leaves less radiation available to photo-initiate the desired chemical reaction.
In addition to exhibiting a greater degree of reflection, short wavelength light also refracts more than longer wavelengths. Greater refraction has two effects that contribute to poor photo-exposure of recording media. The shorter wavelengths lose more energy through lens interference because they have a longer path through the lens material than the longer, less refracted wavelengths. Also, the highly refracted shorter wavelengths of light angle sharply from the focusing lens to the focal point, so light penetration is very shallow. As a result, highly refracted light does not pass through photosensitive material below the surface.
In addition to poor penetration below the surface of the recording material, lasers have other shortcomings as exposure sources in digital imaging. Photosensitive recording materials have polychromatic photosensitivity. Many photosensitive recording media have broad spectral sensitivity, and absorb monochromatic radiation with relatively low quantum efficiency. The photochemical reaction may be initiated with a range of wavelengths, and when only a single wavelength is used, photo-initiation is very inefficient. In order to combat low quantum efficiency, very high intensities are required. High intensity radiation can saturate single absorption bands, which causes failure of the reciprocity law. The practical consequences of absorption band saturation relevant to printing applications include halation, fog and poor resolution in the printed image.
In order to avoid the difficulties with high intensity radiation an ideal light source would have spectral emission similar to the spectral sensitivity of the material. Such spectral overlap could be achieved with an appropriate polychromatic light source. Polychromatic light sources have been used in combination with achromatic lenses to expose photosensitive recording media. Achromatic lenses correct for chromatic aberrations, which are caused from the various wavelengths of light from polychromatic sources. Achromatic optics combine optical elements with low and high indices of refraction to correct for chromatic aberrations. Achromatic lenses focus light with no chromatic aberration so that all the wavelengths of polychromatic light focus at the same focal point. Because achromatic lenses focus all wavelengths of light to a two-dimensional focal point, this exposure method also provides insufficient photo-initiation for the recording material below the surface of the layer.
An illumination method that facilitates polychromatic exposure of a layer of photosensitive material from the surface to the substrate would alleviate many of the aforementioned problems in printing applications.
Polychromatic light sources provide efficient energy in terms of quantum absorption for photosensitive recording media, but achromatic lenses focus the polychromatic light to a single two-dimensional focal point, which prevents exposure of the photosensitive material below the surface of the three-dimensional layer. The present invention takes advantage of the spectral breadth and focal point distribution provided by polychromatic light sources. As a result of these attributes, polychromatic light sources have higher quantum efficiency and permeate deeper below the surface of photosensitive recording media than monochromatic light sources.
Therefore, it is an object of the present invention to provide an illumination method by which polychromatic light intensity can be distributed in a three-dimensional focal point onto recording media.
The above object is obtained by a method for exposing a three-dimensional layer of recording media with an imaging system. The imaging system comprising a polychromatic light source that provides radiation to an optical modulator; the imaging system further including a system of achromatic optics and a chromatic lens along with the layer of recording media.
The method comprises modulating a light beam from the polychromatic light source with the optical modulator and collimating the light beam from the optical modulator with the system of achromatic optics. The method further comprises focusing the light beam from the system of achromatic optics with the chromatic lens and exposing the layer of recording media with the focused light beam from the chromatic lens.
The present invention also provides an imaging apparatus for exposing a three-dimensional layer of recording media. The imaging apparatus comprises a polychromatic light source, an optical modulator, a system of achromatic optics and a chromatic lens. The optical modulator modulates a light beam from the polychromatic light source and the system of achromatic optics collimates the modulated light beam. The chromatic lens focuses the light beam onto the layer of recording media.